The present invention generally pertains to injection molding of hollow plastic products and is particularly directed to methods for reducing the required mold-cavity clamping force and for controlling the wall thickness of a molded product having an end wall and a cylindrical or approximately cylindrical side wall, wherein the end wall includes a fringe region bordering the sidewall.
One prior art, method for injection molding such a product includes the steps of
(a) forming a mold cavity by combining a core section for forming the inside surface of the product with a cavity section for forming the outside surface of the product, wherein the core section or cavity section includes a protractable and retractable ram-shaped contact element, with the end of the contact element that faces the mold cavity including a contact surface; PA1 (b) protracting the contact element to a forward position so that the contact surface contacts the cavity section or core section of the mold cavity respectively; PA1 (c) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position; PA1 (d) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section or core section of the mold cavity respectively; and PA1 (e) injecting a second quantity of plastic material into the mold cavity while the contact element is in the rear position. PA1 (a) forming a mold cavity by combining a core section for forming the inside surface of the product with a cavity section for forming the outside surface of the product, wherein the core section or cavity section includes a protractable and retractable ram-shaped contact element, with the end of the contact element that faces the mold cavity including a contact surface; PA1 (b) protracting the contact element to a forward position so that the contact surface contacts the cavity section or core section of the mold cavity respectively, to secure the core section in position in relation to the cavity section in order to impede movement of the core section in relation to the cavity section; PA1 (c) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position, whereby the wall thickness of the cavity is controlled because relative movement between core section and the cavity section is impeded; PA1 (d) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section or core section of the mold cavity respectively, while the injected first quantity of plastic material at least partly solidifies to impede movement of the core section in relation to the cavity section and thus control the wall thickness of the cavity during further injection of plastic material into the mold cavity; and PA1 (e) injecting a second quantity of plastic material into the mold cavity while the contact elements is in the rear position. PA1 (a) forming a mold cavity by combining a core section for forming the inside surface of the product with a cavity section for forming the outside surface of the product, with the axis of the cylindrical or approximately cylindrical side-wall defining portion of the mold cavity being the same as the mold-opening direction, wherein the core section includes a protractable and retractable ram-shaped contact element, with the end of the contact element that faces the mold cavity including a contact surface; PA1 (b) protracting the contact element to a forward position so that the contact surface contacts the cavity section of the mold cavity; PA1 (c) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position; PA1 (d) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section of the mold cavity; and PA1 (e) injecting a second quantity of plastic material into the mold cavity while the contact element is in the rear position; PA1 (f) forming the mold cavity with a contact element having a contact surface that is able to contact at least a significant part of the fringe region of the end-wall-defining portion of the cavity section; PA1 (g) protracting the contact element to a forward position so that the contact surface contacts at least a significant part of the fringe region of the end-wall-defining portion of the cavity section of the mold cavity, to thereby reduce the mold cavity area as projected on a plane perpendicular to the mold opening direction and thus reduce the clamping force required when the first quantity of plastic material is injected into the mold cavity; and PA1 (h) forming the mold cavity by combining the cavity section with a core section in which both the end-wall defining portion and the side-wall-defining portion of the core section consist of the contact element. PA1 (i) protracting the contract element to a forward position so that the contact surface contacts the cavity section of the mold cavity to secure the core section in position in relation to the cavity section in order to impede movement of the core section in relation to the cavity section; PA1 (j) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position, whereby the wall thickness of the mold cavity is controlled because relative movement between the core section and the cavity section is impeded; and PA1 (k) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section of the mold cavity, while the injected first quantity of plastic material at least partly solidifies to impede movement of the core section in relation to the cavity section and thus control the wall thickness of the cavity during further injection of plastic material into the mold cavity. PA1 (a) forming a mold cavity by combining a core section with a cavity section for forming said product, wherein the core section includes a protractable and retractable contact element, with the end of the contact element that faces the mold cavity including a contact surface; PA1 (b) protracting the contact element along a longitudinal axis of the cavity section to a forward position so that the contact surface contacts the cavity section; PA1 (c) injecting a first quantity of plastic material into the mold cavity while the contact element is in the forward position; PA1 (d) retracting the contact element to a rear position where the contact surface ceases to contact the cavity section of the mold cavity; and PA1 (e) injecting a second quantity of plastic material into the mold cavity while the contact element is in the rear position; PA1 (f) forming the mold cavity by combining the cavity section with a core section in which both the end-wall defining portion and the side-wall-defining portion of the core section consist of the contact element; PA1 (g) forming the mold cavity with a contact element having a contact surface, at least part of which contact surface is non-perpendicular to the longitudinal axis of the cavity section, and which non-perpendicular part of the contact surface is able to contact at least part of the fringe region of the end-wall-defining portion of the cavity section when the contact element is protracted; and PA1 (h) protracting the contact element to a forward position so that the non-perpendicular part of the contact surface contacts at least part of the fringe region of the end-wall-defining portion of the cavity section to secure the core section in position in relation to the cavity section in order to impede lateral movement of the core section in relation to the longitudinal axis of the cavity section.
Also in the prior art, the wall thickness of an injection molded plastic product is controlled by a method including the steps of